A commercial need exists to improve lithography and patterning technology at the micron and nano-scale, including the sub-100 nm scale. Applications include biological applications such as microarrays and nanoarrays for proteins and nucleic acids. Other applications include semiconductor and molecular electronics. In particular, self-assembled monolayers (SAMs) can be a useful strategy for lithography and patterning. An understanding of the factors that control SAM formation and behavior both on the macro- and nanoscopic length scales is important for the technology to reap its full potential. With the advent of molecular deposition tools such as Dip Pen Nanolithography (DPN) printing, it is now possible to study such processes on the nanometer to micrometer length scale (see, for example, Piner et al., Science 1999, 283, 661-663; Ginger et al., Angew Chem Int Edit 2004, 43, 30-45; Hong et al. Science 1999, 286, 523-525; Hong et al., Langmuir 1999, 15, 7897-7900; see also U.S. Pat. Nos. 6,635,311 and 6,827,979) A fundamental issue with such deposition processes pertains to the transport properties of a binary mixture of ink molecules, see for example FIG. 1. For example, the transport of such mixtures could potentially at least result in a nano- or micro structure with a homogenous distribution of adsorbate molecules (FIG. 1, far right), a structure with island-like phase separation (FIG. 1, middle), or a near-complete phase separation of the two adsorbates (FIG. 1, far left).
The commercialization of nanometer-scale mixing of binary monolayers is important for at least two reasons. First, it allows one to elucidate the fundamental properties and origins of phase segregation for two-component mixtures on surfaces. Second, it allows one to deliberately tailor desired surface properties at the sub-50 nm length scale. Applications would include for example DPN printing and microcontact printing (μCP). See, for example, Kumar et al., Science 1994, 263, 60-62; Odom et al., Langmuir 2002, 18, 5314-5320; Gates et al., Annual Review of Materials Research 2004, 34, 339-372.
Surprisingly few examples of nanometer scale phase separation have been reported for binary SAM mixtures (see, for example, Stranick et al., Journal of Physical Chemistry 1994, 98, 7636-7646; Imabayashi et al. Langmuir 1997, 13, 4502-4504; Tamada et al., Langmuir 1997, 13, 1558-1566; Hayes et al. Langmuir 1997, 13, 2511-2518; Jackson et al., Nature Materials 2004, 3, 330-336).
Spontaneous but random phase separation has been observed in scanning tunneling microscope (STM) images of co-adsorbed alkanethiol mixtures on atomically flat gold surfaces (see for example Stranick et al., Journal of Physical Chemistry 1994, 98, 7636-7646; Imabayashi, et al., Langmuir 1997, 13, 4502-4504; Jackson et al., Nature Materials 2004, 3, 330-336; Stranick et al., Nanotechnology 1996, 7, 438-442; Lewis J Phys Chem B 2001, 105, 10630-10636).
It is believed that phase separation of mixed SAMs can be a process which can be driven by polar head group interactions and cohesive interactions between the adsorbate molecules. In such systems, the adsorbates randomly form nanoscale domains with no particular order (see for example Lewis et al., J Phys Chem B 2001, 105, 10630-10636). Phase-separated binary SAMs have been shown useful in resisting non-specific protein adsorption (see for example Jackson et al. Nature Materials 2004, 3, 330-336.), improving DNA hybridization (see for example Satjapipat et al. Langmuir 2001, 17, 7637-7644.), improving the data quality in microarrays (see for example Datwani Langmuir 2004, 20, 4970-4976) and preferential adsorption of cytochrome C (see for example Hobara Nano Lett 2002, 2, 1021-1025.).